The ultimate expression of the genetic makeup of a cell resides in the proteins of that cell. Although protein biosynthesis is a complex process requiring numerous proteins and nucleic acids, the specificity resides, to a large extent, in the first step - the activation and transfer of an amino acid to its respective transfer RNA. Despite the large amount of work devoted to study of aminoacyl-tRNA synthetases and tRNA, relatively little attention has been given to the in vivo mechanism of these reactions, to the structural organization of the macromolecules involved as they exist in the cell, or to the control of protein synthesis that may be exercised at this step. The studies we have proposed aim to answer these questions. Specifically, the objectives of this project are 1) to determine the supermolecular organization of the mammalian protein - synthesizing systems with particular emphasis on the structure and function of the aminoacyl-tRNA synthetase complex; 2) to study and compare purified aminoacyl-tRNA synthetases with those in the complex; 3) to examine the enzymes involved in the biosynthesis and turnover of the -C-C-A terminus of tRNA; 4) to investigate the control of protein synthesis which may be exercised at the level of the biosynthesis of tRNA or on the attachment of amino acids to tRNA. The methods to be used will be enzymological, structural and genetic, and will include the techniques of protein, nucleic acid and lipid chemistry, bacterial and phage genetics, and cellular structure. In view of the central role played by proteins in all cell functions, and the importance of the first reaction of protein synthesis on the specificity of the overall process, an understanding of the mechanism and control of this process is of great importance. Elucidation of the sequence of events, the molecular associations, and the control of protein synthesis in normal cells are a prerequisite to any understanding of abnormal situations which may occur under pathological conditions.